It is well known to distribute air from an air handling system to a main air supply duct to various branch ducts throughout a building. Such buildings may include what is commonly termed "clean rooms," of controlled purity environments. The air flows into the ventilated area and is generally returned via a central return duct back to the air handling system. Naturally, such system could include several systems running in like fashion.
The standard installation of an air handling system, for instance, to a building includes a return air chase or conduit located within the walls or even the floor of the building that extends up above the roof into a generally square or rectangular enclosure. Until the present invention, the square or rectangular enclosure was generally the preferred shape because of the ease of construction and design. Inside the square or rectangular enclosure generally are filter elements, cooling or heating coils, air baffles, noise reduction units, and so forth. Noteworthy, the vast majority of enclosed air handling systems use a centrifugal fan. The centrifugal fans generally have an efficiency of 50-60% and offer an advantage in making a 90 degree turn without the use of vanes and other devices. As an example, U.S. Pat. No. 3,748,997 to Dean, shows the typical installation of a centrifugal fan within a rectangular box. The return air from a central ducting system flows upward into a plenum, turns and then across coils, filters, and so forth, and into the inlet of the centrifugal fan. Outside air may be mixed with the return air (called "makeup air") as the needs of particular installation occur. The blades of the centrifugal fan force the air from the inlet out at a generally 90 degree angle into the supply air duct. Typically, ducting is used to distribute the air from the main supply flow path. Naturally, other combinations can occur. Furthermore, the air ventilation can be used in other aspects as well such as in ducted systems for refrigerated devices, appliances, and electrical instruments. The considerations in designing such a system include noise reduction, volumetric flow (dependent upon static pressure), air efficiency losses, pressure drops, and many other factors. As technology has improved and requirements of filtration heightened (especially in a clean room environment), new ways of performing the old tasks have been sought.
One of the ways in which new ways have been sought is the use of axial flow devices such as axial fans perhaps due to a smaller size, ease of flow control, and higher efficiency. However, while axial fans have been known for many years and find their application in various fields, the designers of central air handling systems have not sought the use of axial fans due to various complications. For instance, with the turns and angular orientations of a typical system, the rotating blades of an axial fan may encounter varying pressure differentials across the flow path. The axial flow fan design is relatively intolerant of unsymmetrical flows across the flow path. While a centrifugal fan, because of its design, generally would not be affected with such pressure differentials, the axial fan could be destroyed by, for instance, breaking a blade with various hazardous effects. Thus, prior to the invention, extensive damage can be done to even large and expensive systems by unsymmetrical flow paths such as a partially blocked filter, large struts, varying flows, and so forth.
Furthermore, a typical axial system, until the present invention, generally requires an extended length of the flow path entering the fan and even to some extent exiting the fan to assist in balancing the flows across the blades of an axial fan. Furthermore, the higher rotational speeds of axial fans may produce high frequency tones that may require careful acoustic design. On the other hand, centrifugal systems generally generate lower frequency noise. Historically, until the present invention, centrifugal fans and its known systems described above such as a square/rectangular enclosure with noise reduction equipment, tend to be more tolerant of non-uniform inlet flows than axial fans and may not require as careful of an acoustic design. For instance, the typical filtering system requires 500 feet per minute (FPM), yet a typical axial flow fan may operate around 3,000 FPM. Thus, the velocity at the filter must be increased without significant distortion. For axial fans, this typically required lengthy straight duct sections in front of the fan to smooth out the distortion and nonuniformity. Again, this is not as important an issue with the centrifugal fan due to the inherent flow path in and through the centrifugal fan.
A hybrid of these airflow systems for buildings involves a "clean room." In clean rooms, high rates of flow are used to essentially purge the air throughout the room. Typically, the air enters a ceiling with a multitude of high efficiency filters, flows vertically at a relatively high flow rate into a floor grill and then is returned to the fan and related duct work. Interestingly, in clean room environments, because of the high flow rates required, multiple units are typically placed over a limited area. However, because of the typical size of air handling systems, this may have resulted in additional support structures and costs to support the extra weight that the present invention may not require. Also, the general state of the art appears to be that the multiple units' return flows are combined in the clean room as the flows enter through the floor grill and are returned to the units via a central collection system. For the clean room environment, multiple filters are used. Some filters are particulate filters. Other filters are chemical absorption filters because, in a given clean room, multiple contaminates may occur. Thus, a typical filter battery of a clean room environment may include a particulate filter and multiple chemical filters to filter the assorted chemicals. This obviously increases the expense with multiple chemicals. Furthermore, some chemical filters may become saturated earlier than other chemical filter elements. Thus, in some cases, the life of the filter element is shortened prematurely as the entire filter bank may be replaced.
In the clean room environment, if different zones are needed in a given clean room, the general state of the art is to divide by walls, partitions, shutters, and so forth, so that different classes of filtration can occur. An example is shown in U.S. Pat. No. 4,699,640 to Suzuki. In the Suzuki reference, the generally accepted philosophy is shown, that is, to divide and physically segregate certain clean air zones and filter the air through high efficiency particulate air filters. It would be convenient and less cumbersome to filter each zone, whether divided or undivided, for that zones' particular contaminant and in particular chemical contaminants.
Thus, while the higher efficiency of an axial fan has been long recognized, the practical ability of those skilled in the art to implement such a system has essentially not occurred until the present invention. Nonuniform flow, noise, high axial velocity, and extended entrance and exit configurations, and other complexities directed the typical air handling system designer and user to specify systems such as might be used on rooftops with centrifugal fans.
Prior to the present invention, no solution offered a combination of features that allowed the efficient and practical use of axial fans in resolving the above difficulties, especially in systems with close turns and without the ability to have lengthy straight duct sections in front of the fan. Actually, the development appeared to be away from the present invention because of the practical difficulties of using axial fans for such air ventilation systems.
Thus, there has been a long-felt unsatisfied need for the invention in allowing for the use of the higher efficiency of axial type flow devices, such as fans, and associated filtration equipment even though the needed implementing arts have long been available. Those skilled in the art appreciated that a problem existed and indeed were unable to arrive at a satisfactory solution. Substantial attempts were made by those skilled in the art, but such attempts failed to set aside the need because they failed to appreciate or understand the problem. Indeed, the prior efforts taught away from the technical direction of the present invention and the use of axial flow devices for air handling systems.